The NSW government must design a 50-year transport plan for Sydney to avoid perpetual gridlock, the state opposition says.

Nationals call for Sydney transport plan

The NSW government must design a 50-year transport plan for Sydney to avoid perpetual gridlock, the state opposition says.

Nationals leader Andrew Stoner, who is also the opposition's roads infrastructure spokesman, said a federal government report warning Sydney faces a 25 per cent increase in traffic congestion in the next 20 years highlights a lack of investment and planning.

"Sydney's heading for perpetual gridlock, thanks to a lack of planning and a failure to invest in key transport infrastructure through Sydney," Mr Stoner told reporters.

The report, commissioned by the federal transport department and based on data from the NSW Roads and Traffic Authority (RTA) and government departments, predicts "high levels of congestion" on Sydney's arterial roads within four years.

"By 2026, the strategic network as a whole is found to be significantly more congested," the report, obtained by The Australian newspaper, said.

The government had misdirected funding into under-utilised infrastructure projects such as the Cross City Tunnel at the expense of necessary wide-ranging initiatives, Mr Stoner said.

"What's needed for Sydney is a 50-year integrated transport plan which incorporates road, rail and ports," he said.

NSW Treasurer Michael Costa dismissed the report as a "load of nonsense".

Record amounts are being spent on infrastructure and the NSW government is unable to spend any more, he said.

"The commonwealth, if it commissions a report, ought to develop some solutions," Mr Costa told reporters.

Meanwhile, Mr Costa has defended the RTA after late finishing road works caused lengthy traffic delays on the F3 Freeway, in Sydney's north, on Monday morning.

Mr Costa, a regular commuter on the freeway connecting the Hunter and Central Coast regions to Sydney, described the situation as "appalling", but placed the blame on contractors.

"I think it was mismanaged, and as I understand it the RTA will be having words with the contractors," he said.